I. Field of the Invention
This invention relates generally to packaging machinery and more specifically to an improved band sealer-type thermal plastic bonding machine for sealing polyethylene film bags or the like where the heating bars and chilling bars utilized therein are laterally and longitudinally movable in a reciprocatory manner relative to the conveyor bands such that greater dwell time for a given machine length can be obtained.
II. Discussion of the Prior Art
In my earlier U.S. Pat. No. 4,080,241 there is described a band sealer for sealing bags formed from thermoplastic films over which the present invention is deemed to be an improvement. As is set forth in that patent, a band-type sealer comprises a framework defining a longitudinal track and first and second endless belts or chains arranged to be driven in opposite directions to one another so that a bag to be sealed inserted at the machine's inlet end of the track is pinched between the conveyor bands and transported through the machine to a discharge point. Disposed between the inlet and the discharge are heating and cooling stations. Additional sets of endless bands are associated with the heating and cooling stations and disposed on opposite sides of the bands along the path of travel of the bag to be sealed are pairs of heating bars and chilling bars, respectively. The heating bars and chilling bars are arranged to abut the moving bands so as to apply heating or cooling, as the case may be, to the zone of the bag to be sealed as it is translated through the heating and cooling stations by the gripping action of the conveyor and sealing bands.
While the invention described in U.S. Pat. No. 4,080,241 constituted a marked improvement over the known prior art at that time in that it reduced considerably the wear and tear on sealing bands due to thermal stress, a certain amount of wear is occasioned by the rubbing contact between the heating and cooling bars and the bands associated therewith.
It has been the practice in the prior art to utilize thin, stainless steel bands at the heating and cooling stations, the bands being coated with a TEFLON material to improve their "release" properties. To reduce the thermal mass of the bands, it has been the practice to fabricate them from relatively thin strips. Repeated heating and cooling of the bands results in stresses tending to weaken them and also tending to distort the bands in such a fashion that they become rippled or corrugated. As such, there is a need for frequent adjustment of the heat transfer bars relative to the bands which, of course, results in machine down-time.
Band sealing machines of the prior art have also utilized TEFLON cloth bands. While they tend to convey heat readily and in a fashion superior to that obtainable with stainless steel bands while providing good release characteristics, such bands do not hold up well when rubbed across the face of the heating and cooling bars.